Four-wheel drive (4WD) vehicles provide traction which is often unattainable in two-wheel drive (2WD) vehicles by delivering power to both of the vehicle's axles. Attendant with the added traction provided by four-wheel drive is the added complexity of the drive train required to control and deliver power to all four wheels as opposed to only two wheels. Four-wheel drive characteristically does not have a center differential which would maintain a fixed fraction of torque delivered to each axle, but rather maintains the same speed at each axle regardless of the torque split between the two axles. The delivery of power between the front wheels and the rear wheels of the vehicle is typically handled by a transfer case mechanism which usually includes either a mechanically- or electronically-controlled clutch to switch between rear-wheel drive and four-wheel drive.
In all-wheel drive (AWD) systems, the front and rear axles are continually in driving engagement with the transfer case, but the system contains a center differential to maintain a fixed ratio of torque between the front and rear axle.
In addition to controlling the transfer of torque between front and rear axles of a vehicle, the transfer case may also include a planetary gear set or set of countershaft gears which provides an underdrive or overdrive ratio. Further, rather than manufacturing a new five- or six-speed transmission, an existing four-speed transmission may be used with a multi-speed transfer case to provide additional speed ratios, such as for stump pulling (extreme underdrive), or in a top gear-overdrive condition.
All-wheel drive transfer cases typically contain a planetary gear set to provide the center differential function that maintains a constant torque split between the front and rear axle independent of tire speed or slip. The vehicle is also typically equipped with identical front and rear axle ratios and identical front and rear tire rolling radii in order to minimize the power (torque related losses) circulated in the center differential. The typical all-wheel drive transfer case only provides a direct or 1:1 torque ratio to the axles, and does not contain any ratio changing clutches because there is only one mode of operation. Some all-wheel drive transfer cases provide a friction clutch across the center differential to dynamically change the front to rear torque ratio.